[PATCH v7 10/11] block/reqlist: implement reqlist_mark_req_invalid()

[Vladimir Sementsov-Ogievskiy]

We do lock qcow2 s->lock only to remove request from the reqlist.
That's quite inefficient. Let's implement atomic operation to avoid
extra critical section.

So new interface is:

1. Instead of reqlist_free_req() user may call atomic
   reqlist_mark_req_invalid().

2. At some moment under mutex user calls reqlist_free_invalid_reqs() to
   free RAM.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/reqlist.h | 13 +++++++++++++
 block/reqlist.c         | 23 ++++++++++++++++++++++-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/include/block/reqlist.h b/include/block/reqlist.h
index 32dc87666f..24d6d93a6e 100644
--- a/include/block/reqlist.h
+++ b/include/block/reqlist.h
@@ -26,6 +26,7 @@
 typedef struct BlockReq {
     int64_t offset;
     int64_t bytes;
+    bool valid;
 
     CoQueue wait_queue; /* coroutines blocked on this req */
     QLIST_ENTRY(BlockReq) list;
@@ -84,4 +85,16 @@ static inline void reqlist_free_req(BlockReq *req)
     }
 }
 
+/*
+ * Invalid request will be ignored when searching for conflicts.
+ * The function modifies .valid atomically and intended for use when we
+ * want to avoid using mutex.
+ * If you use this function don't forget to also call
+ * reqlist_free_invalid_reqs() sometimes, so that list doesn't grow endlessly.
+ */
+void reqlist_mark_req_invalid(BlockReq *req);
+
+/* Remove all invalid requests to free RAM space */
+void reqlist_free_invalid_reqs(BlockReqList *reqs);
+
 #endif /* REQLIST_H */
diff --git a/block/reqlist.c b/block/reqlist.c
index c580752db7..641307d80d 100644
--- a/block/reqlist.c
+++ b/block/reqlist.c
@@ -14,6 +14,8 @@
 
 #include "qemu/osdep.h"
 
+#include "qemu/atomic.h"
+
 #include "block/reqlist.h"
 
 void reqlist_init_req(BlockReqList *reqs, BlockReq *req, int64_t offset,
@@ -22,6 +24,7 @@ void reqlist_init_req(BlockReqList *reqs, BlockReq *req, 
int64_t offset,
     *req = (BlockReq) {
         .offset = offset,
         .bytes = bytes,
+        .valid = true,
     };
     qemu_co_queue_init(&req->wait_queue);
     QLIST_INSERT_HEAD(reqs, req, list);
@@ -33,7 +36,9 @@ BlockReq *reqlist_find_conflict(BlockReqList *reqs, int64_t 
offset,
     BlockReq *r;
 
     QLIST_FOREACH(r, reqs, list) {
-        if (offset + bytes > r->offset && offset < r->offset + r->bytes) {
+        if (r->valid &&
+            offset + bytes > r->offset && offset < r->offset + r->bytes)
+        {
             return r;
         }
     }
@@ -72,3 +77,19 @@ void coroutine_fn reqlist_remove_req(BlockReq *req)
     QLIST_REMOVE(req, list);
     qemu_co_queue_restart_all(&req->wait_queue);
 }
+
+void reqlist_mark_req_invalid(BlockReq *req)
+{
+    qatomic_set(&req->valid, false);
+}
+
+void reqlist_free_invalid_reqs(BlockReqList *reqs)
+{
+    BlockReq *r, *next;
+
+    QLIST_FOREACH_SAFE(r, reqs, list, next) {
+        if (!r->valid) {
+            reqlist_free_req(r);
+        }
+    }
+}
-- 
2.29.2